1. Field of the Invention
The present invention relates to a burst-mode optical receiver and, more particularly, to a burst-mode optical receiver for enhancing an available bit rate in a passive network.
2. Description of the Related Art
For the future generation of communications, optical subscriber networks, such as FTTH (Fiber to the Home), will be required to install optical fiber lines directly to the homes of subscribers for the transmissions/reception of information at a higher speed. The subscriber networks have been traditionally constructed with copper-based lines. As such, it would be very costly to replace them with the fiber-based lines. In this regard, a passive optical network (PON) has been taken into consideration to provide a more cost-effective optical-subscriber network.
FIG. 1 illustrates a general PON system. As shown in FIG. 1, the PON is generally made up of an optical line termination (OLT) located in a central office, a 1×N passive optical splitter, a plurality of optical-network units (ONUs) corresponding to each subscriber. In this type of optical multi-access network, each node is designed to transmit data or packets to other nodes using a predetermined time slot. Typically, a plurality of subscribers can make use of a single optical line through which desired data are transmitted or received according to a time-division multiplexing scheme. Unlike the point-to-point link, burst-mode data are generated in which received data or packets have different sizes and phases from each other due to the optical loss or attenuation generated via different transmission routes. Each subscriber transmits data at the respective assigned time, but the packets received at the receiving ends are not uniform in size due to the path differences between the OLT and each subscriber.
As each received packet has a different size and phase due to the optical loss or by different transmission routes between the nodes, an optical receiver must be employed to compensate the loss. To this end, a burst-mode optical receiver is used to enable the received packets to have the same sizes and phases. The conventional burst-mode optical receivers prevent the loss of burst data caused by a charging/discharging time of the capacitor in the receiver by removing the DC block capacitor. A threshold value is extracted from each received packet by the receiver which functions as a reference signal for the purpose of data discrimination, and the data is amplified using the extracted discrimination reference signal.
For example, FIG. 2 is a circuit diagram of a conventional burst-mode optical receiver. The burst-mode optical receiver of FIG. 2 includes an optical detector 1 for converting input optical signals into current signals, and a trans-impedance amplifier (TIA) 2 for converting current signals passing through the optical detector 1 into voltage signals. Note that the TIA 2 is dc-coupled. Signals received by the optical detector 1 are amplified at the TIA 2 and then divided into two parts, of which one is dc-coupled to and inputted into a differential amplifier of a limiter amplifier 4 and the other is inputted into a circuit for an automatic threshold controller (ATC) 3. The ATC 3 extracts discrimination thresholds of the respective packets received from the TIA 2. The limiter amplifier 4 amplifies signals with a different optical intensity into signals having a constant amplitude using the extracted discrimination thresholds. The thresholds that vary according to the sizes of packets outputted from the ATC 3 are inputted into an input terminal as a reference voltage Vref of the differential amplifier of the limiter amplifier 4 to be amplified and recovered.
FIG. 3 is a circuit diagram of another conventional burst-mode optical receiver having a structure with a differential input/output feedback amplifier. The optical receiver of FIG. 3 includes an optical detector 8, a differential preamplifier 10, a peak detector 20, and a limiting amplifier 30. The peak detector 20 detects the peak value of an output signal to generate a reference voltage so as to set the discrimination thresholds of received packets. The limiting amplifier 30 amplifies recovered signals using the generated reference voltage. The differential preamplifier 10 is operative to receive current signals, which are detected at the optical detector 8, as inputs and then outputs corresponding voltages. A ratio of the input current to the output voltage, i.e., a trans-impedance, is determined by a feedback resistor ZT. One side of the feedback resistor ZT is connected to a “+” input terminal of amplifier 12 and the other is connected to a “−” output terminal of amplifier 12. The peak detector 20 is made up of an amplifier 22, a drive transistor 24, a buffer transistor 26, a charging capacitor CPD, and a bias circuit 28. Here, a reference voltage Vref, which is outputted from the peak detector 20, is converted into a discrimination-threshold current by the feedback resistor ZT.
During operation, the “+” input terminal of the amplifier 12 receives the current IIN outputted from the optical detector 8, and the “−” output terminal receives the reference voltage Vref or a reference signal. Here, the reference signal inputted to the “−” output terminal is a discrimination-threshold current converted from the reference voltage Vref, which is detected from the peak detector 20. Accordingly, the differential preamplifier 10 generates output voltages Vo+ and Vo− depending on the difference between the two input currents.
The output voltage Vo+ outputted from the “+” terminal of the amplifier 12 in the differential preamplifier 10 is inputted to a “+” terminal of an amplifier 22 of the peak detector 20, whereas the reference voltage Vref applied to the “−” terminal of the amplifier 12 of the differential preamplifier 10 is fed back to a “−” terminal of the amplifier 22 of the peak detector 20. Therefore, when these two voltages are not the same at the amplifier 22 of the peak detector 20, the drive transistor 24 is turned on and causes the charging capacitor CPD to be charged with voltage until the “+” and “−” terminals of the amplifier 22 have the same voltage. Accordingly, when an optical-detection signal, first input IIN, flows into the differential preamplifier 10, its output becomes ΔVo+=ΔVo−. Further, as the peak detector 20 is supplied with the output of ΔVo+ at its “+” terminal, the voltage charged at the charging capacitor CPD becomes the reference voltage Vref. This reference voltage Vref is used as a threshold for discriminating data using a mean level of an output-data signal.
Meanwhile, when the two voltages are the same at the amplifier 22 of the peak detector 20, the drive transistor 24 is turned off, and thus the charging capacitor CPD is discharged. With this discharge, the buffer transistor 26 is turned on, and thus the current flows through the bias circuit 28. Thereafter, the reference voltage Vref is applied to a node between the buffer transistor 26 and the bias circuit 28 and then converted into a discrimination-threshold current by the feedback resistor ZT, and finally fed back to the “−” terminal of the amplifier 12 of the differential preamplifier 10. Thus, the current flowing to the “−” terminal of the amplifier 22 of the peak detector 20 corresponds to a middle value of the optical-detection signal IIN current. Hence, the reference signal Vref functions as the discrimination threshold of the differential preamplifier 10.
However, the actual reference signal Vref is typically accompanied by an offset of the differential preamplifier 10, resulting from device asymmetry as well as a structural offset caused by the turn-on voltages of transistors resulting from a circuit structure of the peak detector 20. Thus the actual reference signal tends to deviate from a mean or middle level of the output data signal. A pulse width distortion is generated due to the change in the reference signal which in turn degenerates the sensitivity of the optical detector 8.
To minimize this pulse-width distortion, the conventional feedback burst-mode optical receiver employs a current source IADJ, which is connected to the “+” input terminal and the resistor ZT of the differential preamplifier 10. The current source IADJ serves to compensate the offset generated by the differential preamplifier 10, but does not compensate the structural offset generated by the turn-on voltages of the transistors within the peak detector 20.
Accordingly, there is a problem in that the reference signal generated from the peak detector 20 is not matched with the mean level of the output-data signal, thus still generates a pulse-width distortion and degrades the sensitivity of the optical detector.